The invention relates to a frequency descriminating device which can readily discriminate two signals having similar frequencies.
Heretofore, in order to discriminate two signals having closely adjacent frequencies f.sub.1 and f.sub.2, filters having passbands which pass only the specific frequencies were employed, or the phase characteristic of a filter was utilized. For instance, a circuit as shown in FIG. 1 was used. In FIG. 1, reference numeral 1 designates a low-pass filter, 2 a phase comparator, and a and b an input terminal and an output terminal, respectively.
In the circuit, the signals to be discriminated are applied directly to the phase comparator 2 and applied through the low-pass filter 1 (which may be a high-pass filter) to the phase comparator 2. As is apparent from the characteristic curve of phase versus frequency as shown in FIG. 2, the phase conditions at the frequencies f.sub.1 and f.sub.2 are different. Accordingly, the signals applied to the phase comparator 2 through the low-pass filter 1 are different in phase from the original signals. This variation in phase is detected as an output by the phase comparator 2.
In FIG. 1, the low-pass filter 1 is, for example, a two-pole low-pass filter which rolls off at -12 dB/oct. Therefore, in that case, for the frequencies f.sub.1 and f.sub.2, discrimination can be achieved by phase detection of 0.degree. and 180.degree.. However, the Q characteristic of the low-pass filter 1 must be steep, otherwise the phase variation will be small and accordingly the discrimination rather difficult. On the other hand, if the frequencies f.sub.1 and f.sub.2 are very close to each other, it is difficult to sufficiently accurately set a cut-off frequency for the low-pass filter. If the drift of the circuit elements is taken into account, then it is often impossible to employ this technique.
The present invention further relates to a frequency discriminating device which discriminates an amplitude-modulated signal (hereinafter referred to as "AM") using one of two adjacent frequencies, as in the case of a control signal for television sound multiplex broadcasts.
A conventional frequency discriminating device of this type is shown in FIG. 3. In FIG. 3, reference numeral 11 designates an input terminal to which is applied an AM input signal modulated by one of two adjacent frequencies, 12 a limiter connected to the input terminal 11, 13 a frequency divider connected to the limiter 12, 14 a frequency mixer which receives the AM input signal and the output signal of the frequency divider 13, 15 a first low-pass filter for obtaining the difference component between the output signal of the freuquency divider 13 and the modulation frequency of the AM input signal, 16 a second low-pass filter, 17 a phase comparator, and 18 an output terminal.
The device thus constructed operates as follows: It is assumed that the carrier frequency of the AM input signal is represented by f.sub.c and the modulation signal of the AM input signal is represented by f.sub.1 or f.sub.2. Upon applying the AM input signal to the limiter 12 and the frequency divider 13, a reference signal is obtained. The frequency of the reference signal can be represented by the following expression: EQU f.sub.m =f.sub.c /N, (1)
where N is the frequency division ratio of the frequency divider.
After the AM input signal and the reference signal are applied to the frequency mixer 14, only the difference component between the two signals is obtained at the output of the first low-pass filter 15. Therefore, the output signal of the first low-pass filter 15 has a frequency component f.sub.1 -f.sub.m or f.sub.2 -f.sub.m.
This output signal is applied to the second low-pass filter, which has a characteristic as shown in FIG. 4. A phase inversion between the signals .vertline.f.sub.1 -f.sub.m .vertline. and .vertline.f.sub.2 -f.sub.m .vertline. is performed by the second low-pass filter 16 due to its having a cut-off frequency f.sub.o between .vertline.f.sub.1 -f.sub.m .vertline. and .vertline.f.sub.2 -f.sub.m .vertline.. Thus, by applying the output of the first low-pass filter 15 directly to the phase comparator 17 and by applying the same through the second low-pass filter 16 to the phase comparator 17, it can be discriminated whether the modulation frequency is f.sub.1 or f.sub.2.
For instance, in an actual television sound multiplex broadcast (as used in Japan), the carrier frequency f.sub.c is 55.125 kHz and the modulation frequency is 982.5 Hz (f.sub.1) for stereo broadcasts and 922.5 Hz (f.sub.2) for split-channel (bilingual) broadcasts. If the frequency division ratio is 61, then the reference signal f.sub.m has a frequency of 903.7 Hz. Thus, .vertline.f.sub.1 -f.sub.m .vertline. and .vertline.f.sub.2 -f.sub.m .vertline. are 78.8 Hz and 18.8 Hz, respectively. (If N is 55, then f.sub.m is 1002.3 Hz, and therefore .vertline.f.sub.1 -f.sub.m .vertline. is 19.8 Hz and .vertline.f.sub.2 -f.sub.m .vertline.=79.8 Hz as opposed to the above-described case.) The first low-pass filter 15 is set to pass a frequency band lower than 100 to 200 Hz. In the case of the frequency division ratio N=61, the signal at 78.8 Hz for stereo broadcasts and the signal at 18.8 Hz for bilingual broadcasts are discriminated. If the cut-off frequency f.sub.o of the second low-pass filter 16 is set to 40 to 50 Hz, then these signals can be clearly discriminated by the phase comparator 17. Since the second low-pass filter 16 is utilized only for phase inversion, it is evident that the same effect can be obtained by using a high-pass filter.
However, in the conventional device, not only a phase inversion but also a signal level drop takes place on the f.sub.1 -f.sub.m side in the second low-pass filter as shown in FIG. 4. Therefore, the conventional device suffers from a drawback in that, if an AM input signal having a low S/N ratio because of low received electric field strength, etc. is applied, it cannot be clearly discriminated.
In a further example of the prior art shown in FIG. 5, a selective amplifier 41 amplifies only an AM signal at 55.125 kHz, and the signal at 922.5 Hz or 982.5 Hz is obtained through an AM detector 42. The signal thus obtained is applied to a reed filter 43a or 43b to detect the frequency thereof. A level detector 44a or 44b provides an output.
The output as a result of AM detection includes a great deal of noise, and the difference between the frequencies 992.5 Hz and 982.5 Hz is only 60 Hz. Therefore, the filters must be have excellent and stable frequency characteristics. Accordingly, reed filters are employed. However, even reed filters does not have fully acceptable response characteristics and vibration withstanding characteristic due to the mechanical construction thereof.
In view of the foregoing, an object of the invention is to provide a frequency discriminating device having a wide range of applications.